Electrical machining techniques utilize an electrode spaced closely adjacent a workpiece so that pulsed electrical current is passed between the electrode and the workpiece through a small gap, thereby machining the workpiece. A series of time-spaced, discrete electrical discharges created across the machining gap thus erosively remove material from the workpiece. The high current density discharges across the gap commonly occur at sonic or ultrasonic frequencies and in the presence of a dielectric fluid which fills the gap during the machining process. More particularly, electrical machining generally includes electrical discharge machining (EDM), electrochemical machining (ECM), electrochemical-discharge machining (ECDM), and other machining processes which utilize electroerosive techniques to machine material from a workpiece.
U.S. Pat. Nos. 3,318,793 and 3,610,865 disclose electrical machining techniques developed in the 1960s and the 1970s. The '793 patent discloses an electrohydraulic servo feed system which allegedly has advantages over a hydraulically actuated servo feed system for moving the electrode with respect to the workpiece. An EDM cutting machine which immerses the electrode in a liquid bath is disclosed in the '865 patent. U.S. Pat. No. 4,439,659 discloses apparatus for electrical machining a work surface. The system includes a gap-sensing circuit with the electrode being emersed in a liquid dielectric bath, and a servo motor for moving the electrode.
Precision notches may be cut in a oilfield tubular for the purpose of calibrating ubular inspection equipment. Specifics regarding notches for calibrating the inspection equipment are standardized by the American Petroleum Institute (API), and each notch generally has a length substantially greater than the width. Both the depth of the cut notch and the length and width of the notch are carefully controlled in order to reliably calibrate both electromagnetic and sonic inspection equipment. Also, API specifications have mandated that notches be cut at different orientations with respect to the axis of the tubular, with some of the notches being aligned with the axis of the tubular, other notches being traverse to the axis of the tubular, and still other notches being at preselected angles with respect to the axis of the tubular.
U.S. Pat. No. 4,162,383 discloses a machine for cutting the inside of a tubular utilizing oxy-arc techniques. A plurality of claws and rollers are provided for positioning the equipment within the tubular. U.S. Pat. No. 4,476,368 discloses a disintegrator arc tool for cutting the interior wall of a tubular as the tool moves an electrode toward and away from a work surface. Electrical voltage is supplied to the electrode for performing the electrical machining operation, and a non-conductive emersion fluid is supplied for filling the gap between the electrode and the work surface. U.S. Pat. No. 4,948,933 discloses another machine for cutting the interior surface of a tubular. An articulated arm mechanism and a force rod are disclosed in one embodiment for selectively positioning the electrode within the tubular, while a hydraulic technique involving a master cylinder and a slave cylinder are disclosed in an alternate technique for positioning the electrode within the tubular. The electrode may be rotated to any desired angular relationship with respect to the axis of the tubular. In one embodiment, a jaw mechanism is provided for releasably gripping the electrode. In another embodiment, a biasing spring and detent balls are provided for allowing the electrode to move relative to its support.
When used for cutting precision notches on the interior or the exterior of an oilfield tubular, the above equipment is expensive to manufacture and difficult to maintain. The equipment is also complex, and a good deal of training and expertise is required to properly operate the equipment. When the surface of the tubular to be precision cut is irregular or slightly corroded, the tool may not cut the desired API specified notch in the tubular. Accordingly, the notch cutting and calibration process may have to be repeated several times. Particularly when the notch is cut in the interior of the tubular, the operator may not be able to determine that an improper notch has been cut, and accordingly the test equipment calibration may be incorrect. Even with a proper API notch cut in an oilfield tubular surface by an experienced operator, a large amount of operator time is required to cut the notch, thereby increasing the cost of the inspection calibration operation.
The disadvantages of the prior art are overcome by the present invention, and improved equipment for cutting precision notches in both the interior surface and exterior surface of an elongate metal tubular is hereinafter disclosed. Those familiar with prior art equipment and techniques for cutting precision notches in tubulars have long sought and will appreciate the advantages of the present invention, as hereinafter disclosed.